The manufacture of semiconductor devices uses a lithographic, technique. It is often necessary to align or locate an object with a known reference coordinate system. In microcircuit lithography, the image of a reticle is projected onto a photosensitive substrate or wafer which is reproduced forming circuit patterns. Different processing steps often require different layers to be sequentially formed on the substrate to form a semiconductor device. Accordingly, it is often necessary to relocate the substrate with a high degree of accuracy. The substrate must be accurately positioned relative to prior patterns formed thereon. Alignment accuracy is often critical and typically is required to be less than 0.1 micron. Generally, an alignment mark or target is placed on a substrate to be aligned and is located with reference to a second object such as an alignment sensor. The position of the alignment mark, and therefore the substrate, is determined with respect to the alignment sensor coordinates. This is a critical step in alignment. The position of an alignment mark is normally defined with respect to the wafer plane and the center of symmetry of some portion of the alignment mark pattern. Typically an alignment sensor has an alignment axis that is normal to the plane of the substrate or wafer. Generally, alignment sensors require the use of a special alignment mark pattern, or a relatively restricted class of alignment mark patterns, that is placed on a substrate or wafer.
An alignment system is disclosed in U.S. Pat. No. 5,477,057 entitled “Off Axis Alignment System For Scanning Photolithography” issuing to David Angeley et al on Dec. 19, 1995, which is herein incorporated by reference. Therein disclosed is an alignment system having multiple detectors for detection of light scattered and reflected from alignment marks placed on a wafer. Another alignment system is disclosed in U.S. Pat. No. 5,559,601 entitled “Mask And Wafer Diffraction Grating Alignment System Wherein The Diffracted Light Beams Return Substantially Along An Incident Angle” issuing to Gallatin et al on Sep. 24, 1996, which is herein incorporated by reference. Therein disclosed is a grating-grating interferometric wafer alignment system utilizing a coherent illumination for determining alignment from the return electromagnetic radiation intensity. Another alignment system is disclosed in U.S. Pat. No. 5,767,523 entitled “Multiple Detector Alignment System For Photolithography” issuing to McCullough on Jun. 16, 1998, which is herein incorporated by reference. Therein disclosed is a detector having a photosensitive surface covered with an opaque layer having predetermined openings therein combined with a matched reticle having predetermined apertures.
While these and other alignment systems have performed adequately for their intended purpose, there is an increasing need for improving the accuracy of alignment systems. Additionally, there is a need for an alignment system that can utilize a relatively large class of alignment mark patterns.